1. The Field of the Invention
The present invention refers to an optical glass, the use of such an optical glass, optical elements or preforms, of such optical elements, a method for the production of the optical elements and optical parts or optical components made of such optical elements.
2. The Description of the Related Art
Conventional optical glasses of the optical position claimed herein (preferably dense barium flint position as well as border regions of the flint, dense flint, light flint and barium flint position) for the application areas of imaging, sensor technology, microscopy, medical technology, digital projection, photolithography, laser technology, wafer/chip technology as well as telecommunications, optical communications engineering and optics/illumination in the sector automotive usually comprise lead oxide (PbO) in order to achieve the desired optical properties, i.e. refractive index of nd between 1.60≦nd≦1.72 and/or an Abbe number vd of between 32≦vd≦45. Lead oxide is also often used to adjust a pronounced short flint character. This leads to glasses with poor chemical resistance. Further, the component lead oxide is harmful to the environment.
Additionally As2O3 is often used as a fining agent. In recent years, next to PbO also As2O3 is regarded harmful to the environment and therefore most suppliers of optical instruments and products tend to use glasses that are free of lead and arsenic oxides. For use in high value products, glasses with increased chemical resistance gain increasing importance.
Known lead free glasses of this optical position are usually based on the use of high amounts of TiO2 in silicious matrix, which on the one hand leads to glasses with a high tendency to crystallize, which can therefore not be subject to secondary hot moulding steps, and on the other hand leads to glasses, which are to difficult to handle in mechanical further processing steps, because of their increased hardness.
In optical systems of high quality the correction of colour aberrations is an important issue already during system design. For excellent correction of colour aberration glasses with pronounced short flint character are needed. Those are glasses with a relative partial dispersion significantly deviating from the normal line and, thus, are especially suited for correction of colour aberration. However, this property is often achieved by application of PbO, which should not be used according to the above-mentioned ecological concerns.
Instead of conventional detaching of optical components from block or ingot glass production methods are becoming more and more important, in which optical components are obtained by precise pressing, i.e. directly pressing optical components and/or near-netshape preforms for secondary moulding processes—so-called precision gobs—after melting the glass. This is true for the consumer market as well as for the high quality market. “Precision gobs” are usually and preferably understood to be fire-polished, free or semi-free moulded glass portions obtainable by different production processes.
The above-described is the reason for a strong demand for “short” glasses in process engineering, melting and hot moulding. Glasses are considered “short”, if their viscosity very strongly changes with temperature. This behaviour offers the advantage of reproduced duration of hot moulding processes and, thus, reducing the moulding duration in near-netshape precision moulding. Thereby on the one hand the throughput and the efficiency are increased and on the other hand the material of the moulding tools is preserved. This has strong positive impact on the overall production costs. Furthermore, the quick solidification of short glasses allows for the processing of glasses with stronger crystallization tendencies and the formation of seed crystals, leading to problems in secondary moulding steps, is avoided or at least strongly decreased.
Based on the same considerations, there is a need for glasses, the temperature viscosity profile of which shows—absolute—low temperatures during hot moulding. This additionally contributes to decreased process temperatures and thereby allows for improved unit lifetime and by quicker tension-less cooling contributes to reduced formation of seed crystals. Additionally, these glasses provide for an important broader range of possible, probably more cost-effective moulding tool materials, which is particularly important in near-netshape precise pressing.
In modern high performance optics, higher requirements have to be met as to the imaging precision and resolution. That means that on the one hand growing imaging and projection areas have to be obtained and on the other hand the structures to be imaged become smaller and have to be depicted even more precisely and detailed. Thus, it is necessary to irradiate with light of shorter wavelength, i.e. with higher energy, which increases the energy-based load of the optical elements. Additionally, in order to increase production speed, shorter light exposure is aimed at in a multitude of technical applications, like for example microlithography, thus leading to higher radiation power and density, being transmitted through the optical system, thereby necessarily increasing the radiation load per time. Still further in optical systems, especially in telecommunications and communications engineering a high light efficiency, i.e. a high transmission, is needed.
This is not only a challenge to optical system design, but also to the development of optical glass for such optical applications. For example it is known that application of higher radiation densities leads to a phenomenon known as solarization, i.e. a radiation mediated change in structure within the glass, further leading to a strong decrease in transmission, i.e. permeation of light through the glass, of an optical element. Therefore, glasses are needed that show strong resistance against the solarization phenomenon.
The following documents concern glasses according to the state of the art:                JP 87-012633A (Hoya)        JP 77-069915A (Hoya)        JP 58-120539A (Ohara)        U.S. Pat. No. 5,007,948 (Corning)        
According to these documents there are glasses obtainable that have similar optical positions and chemical compositions, but these show significant disadvantages when compared to the glasses according to the present invention.
The glasses disclosed in the laid-open patent application JP 87-012633A comprise significant amounts of more than 13 mol % Cs2O, which is very expensive in highly pure qualities. On the one hand this oxide serves as an adjusting agent for a refractive index gradient in interaction with the exchange partner ZnO, on the other hand the high amount of flux melting agent weakens the matrix in order to increase ion mobility such that a meaningful exchange followed by accelerated and effective relaxation can take place in order to avoid tension within the material.
According to this document, furthermore lower positions in terms of refractive indices are aimed at; therefore proportions of ZrO2 and Nb2O5 are present only optionally and in small amounts (ZrO2 smaller than <4 mol %; Nb2O5 smaller than 1 mol %).
Because of similar reasons, the glasses disclosed in JP 77-069915A show corresponding disadvantages: due to the small amounts of ZrO2, the desired optical position especially a high dispersion of vd≧32 cannot be achieved. The high amounts of CaO especially in combination with high possible amounts of other alkaline earth metal oxides serve as matrix modifier and, thus, act as destabilizers and promoters of crystallization towards the glasses.
JP 58120539 discloses glasses with high contents of silicon dioxide, all of which glasses comprise the expensive component lithium oxide, while boron oxide is optionally present. By application of the already more expensive lithium oxide, the possible application of boron oxide as matrix stabilizer is strongly limited, as the combined presence of both components, boron and lithium oxide, exerts synergistic attack on the fire proof material.
The glasses disclosed in U.S. Pat. No. 5,007,948 comprise relevant proportions of silver for adjusting a refractive index gradient. Thereby the glasses not only become uneconomical, they also are redox-sensitive during hot moulding. They thus loose their workability as far as secondary moulding steps are concerned, like for example re-pressing, precise pressing of lenses and similar. Also actions, such as tension and target cooling processes, as usually applied to classical optical glasses, leads to undesirable properties. Furthermore, the basic glasses according to this document even before ion exchange comprise large amounts of Al2O3 that facilitates ion exchange. By this additional amount of high melting components (up to 32 cation mol %) the glasses become difficult-to-melt and thereby even less economical on the one hand and on the other hand show loss of transmission at the blue spectral edge because of the higher melting temperatures leading to impurities from platinum crucible material. As a possibility for compensation of these disadvantages, large amounts of alkaline metal oxides Na2O are added, up to 39 cation mol %, i.e. a flux melting agent. Thereby a structure in the material is obtained that is facilitating ion exchange but is detrimental as to the stability towards crystallization, which is not desirable.